Various magnetic centre-finding structures are known. They generally rely on the use of permanent magnets, windings, ferromagnetic armatures and, more often than not, an electric excitation circuit for controlling the magnetic fluxes generated by the windings. The role of a magnetic centre-finding bearing is to centre a moving body relevant to a reference body. A plurality of movements of the moving body relative to the reference body is thus prevented, or controlled. If we consider an orthogonal frame of reference consisting of three axes X-X, Y-Y, and Z-Z, centred on the centre of the device including the magnetic centre-finder, the moving body and the reference body, the axis Z-Z forming an axis of revolution of the device, and the axes X-X and Y-Y defining a median plane of the device, there are then three translations along the axes X-X, Y-Y and Z-Z and three rotations about these three same axes.
Different types of magnetic centre-finding bearings can be used to control the three translations and the three rotations, passively and/or actively.
Moreover, the moving body retains at least a degree of freedom relative to the reference body, generally a rotation about the axis Z-Z; in this case, the moving body is called rotor and the reference body stator.
Generally, tilts along the other axes are on the other hand controlled only passively. More specifically, it is in order to minimize the number of active axes, because this requires electronic controls, that it is preferable to manage the tilts passively.
Some magnetic centre-finding bearings therefore make it possible to control the three translations of the moving body relative to the reference body: along one active axis and two passive axes, along two active axes and one passive axis, or along three active axes.
Currently, for a magnetic centre-finder to be testable on the ground, under gravity, in any position, without consuming extra energy in order to sustain the rotor, the presence of magnets is necessary. It indeed makes it possible to sustain the latter under gravity without the addition of extra energy. In the absence of these magnets, it would be necessary for the magnetic bearings of the centre-finder to control five active axes, which would greatly increase the complexity of the equipment and its electronics. Furthermore, the design of a “magnetic centre-finder—rotor with magnets” subassembly is very difficult because of magnetic constraints, such as the need not to place glue on the poles of the magnets in order to minimize and master the air gaps, and mechanical constraints, such as resistance to vibration stresses and centrifugal stresses.
The current solutions that seek to propose magnetic centre-finders that can be tested in all positions under gravity, come up against the abovementioned difficulties. They include either magnets on the rotor, as in the example of FIG. 1, or magnets on the stator, but in this case, they also include an additional non-functional air gap, called static air gap. Worthy of mention are the French patents FR88400586 and FR8703489, or even EP0724 086 and U.S. Pat. No. 4,043,614. In the first case, the subassembly with the rotor is complex and constraining. It comprises a mechanical structure on which the magnets are assembled. In order to ensure the mechanical withstand strength, in particular to the vibratory stresses, such equipment generally requires protective crowns or spacers to contain the magnets subjected to the centrifugal force. Moreover, they include fixing means such as screws or glue. In the latter case, the windings exhibit a reduced efficiency because of the static air gap described previously.
Among the known technologies of the state of the art, the patent EP0284 487 can be cited. However, the device described in this patent necessarily includes, in addition to the functional air gaps participating in the generation of a magnetic flux, static, non-functional air gaps. The role of this static air gap is to prevent the whole of the magnetic flux generated by the magnets from passing into the coil, to the detriment of the rotor.
This static air gap requires the coil to be overdimensioned, since it does not participate in creating the flux between the rotor and the stator. It exists only through physical necessity. If the magnetic flux generated by the magnets does not pass to the rotor, the centre-finder would not function. However, the wider this static air gap becomes, the more bulky the coil needs to be because the magnetic flux from the coil also passes through this air gap.
Similarly, the device described in the patent EP0613 594 necessarily includes static, non-functional air gaps.
The U.S. Pat. No. 4,652,780 describes a magnetic centre-finding device in which the magnetic circuit followed by the magnetic flux generated by the windings does not pass through the same magnetic circuit as that of the magnets. This notably means:                a greater total mass because the device includes a plurality of magnetic circuits, one “long” circuit of which bypasses the other to be looped back;        greater iron losses because of the long magnetic circuit.        
These magnet-based magnetic bearings are moreover difficult to control, partly because of a delay phenomenon induced by these iron losses.
Yet other technologies have been developed, but all have the drawback of resulting in overdimensioned magnetic centre-finding devices, that is to say devices that have a non-optimized bulk.
It is to overcome these drawbacks that the present patent application proposes a magnetic centre-finding concept with no magnet on the rotor.